CONSTRAINING CAUSES OF STRUCTURAL FAILURE USING ELECTRICAL RESISTIVTY TOMOGRAPHY (ERT): A CASE STUDY OF LAGOS, SOUTHWESTERN, NIGERIA

Abstract

The phenomenon of building collapse in recent times has become a source of national concern. Particularly alarming is the situation in Lagos, the commercial nerve center of Nigeria. Several attempts made to proffer solution to this excluded geophysical investigations, as general opinion did not support subsurface geology as being responsible. However, 2-D and 3-D electrical resistivity surveys were carried out using wenner electrode configuration with 64 electrodes connected to a multi-core cable, along three (3) profiles of 260 m length at an inter-electrode spacing of 4m. Resistivity measurements were taken alongside with Induced Polarization measurements using the same electrode configuration. The survey was conducted round to cover all the areas where distress in building structures is noticeable. The 2-D and 3-D electrical resistivity and IP images for the three profiles revealed that the main cause of structural defect in buildings around the area is the subsurface geology, contrary to general opinion which favours insufficiency and/or lack of genuine building materials. From the results, it is quite obvious that the entire land mass of the study area is underlain by materials of very low resistivity values below 30 Ωm at the near-surface depth of 5 m down to above 30 m. Local high resistivity in the near surface material is due to either the effect of sandfilling for road network and/or the presence of exotic highly resistive concrete materials used to reclaim land. The IP inversion models help to delineate clay formation from sandy formation filled with saline water. The high IP signals in the subsurface mainly confirm the presence of clayey formation.